1. Field of the Invention
The present invention relates to an ink jet recording apparatus including record means having a plurality of discharge ports (or discharge orifices) arranged therein for discharging ink to a recording medium to record information.
2. Related Background Art
Office automation equipments such as computers, word processors and copiers have recently been widely used and a number of recording methods of recording apparatus therefor have been developed. An ink jet recording apparatus is characterized in that it is easier to attain fine recording with such an apparatus than other recording method, higher speed and more silent, and is less expensive. A need for the ability to print in color is also increasing and many color ink jet recording apparatus have been developed. The ink jet recording apparatus discharges ink from a nozzle to deposit the ink to a record sheet to form an image. In order to enhance a recording speed, a recording head (multi-head) having a plurality of recording elements integrated therein is used so that a plurality of ink discharge ports and ink paths are integrated. For color printing, a plurality of such multi-heads are arranged.
However, unlike a monochromatic printer for printing only characters, various factors such as hue (coloring or color development), tonality and uniformity must be considered in printing a color image.
As to the uniformity, even a slight variation among nozzle units due to a difference among multi-head manufacturing processes affects to an amount of ink discharged from the nozzle and a direction of discharge in printing and it finally appears as ununiformity of density in the printed image, which causes a degradation in image quality.
A specific example is explained with reference to FIGS. 25 and 26. In FIG. 25A, numeral 91 denotes a multi-head which is assumed to comprise light multi-nozzles 92. Numeral 93 denotes an ink droplet discharged from the multi-nozzle 92. Ideally, the ink is discharged with a uniform amount of discharge in a uniform direction as shown in FIG. 25A. If such discharge is attained, dots of uniform size reach a record sheet as shown in FIG. 25B so that a uniform image free of ununiformity in density is formed overall (FIG. 25C). However, in reality, there is a variation among nozzles as described above, and if the printing is done in the manner described above, there occurs variations in the size and direction of the ink droplets discharged from the nozzles, and the droplets reach the record sheet in a manner shown in FIG. 26B.
As seen from FIG. 26B, a white area which does not meet an area factor of 100% appears periodically relative to a head main scan direction, or dots overlap more than required, or a white band appears as shown at the center of the drawing. An aggregation of the dots deposited on the record sheet has a distribution of density in the direction of nozzle arrangement as shown in FIG. 26C, and it appears as ununiformity of density when it is observed by human eyes.
The following method has been proposed as a countermeasure for the ununiformity of density. It is explained with reference to FIGS. 27 and 28. In this method, the multi-head 91 is scanned three times to complete the print area shown in FIGS. 25 and 26 but half of the print area (four-pixel unit area) is completed in two passes. The eight nozzles of the multi-head are divided into an upper four-nozzle group and a lower four-nozzle group, and the number of dots to be printed by one nozzle in one scan is reduced to approximately one half of the dots of the given image data in accordance with a predetermined image data arrangement. In the second scan, dots are printed in accordance with the remaining half of the image data to complete the printing of the four-pixel unit area. The above recording method is hereinafter referred to as a divisional recording method.
In accordance with this recording method, the affect of the nozzle inherency to the printed image is reduced to one half even if the same multi-head as that shown in FIG. 26 is used, and the printed image appears as shown in FIG. 27B in which the black stripes and the white stripes shown in FIG. 26B are not very prominent. As a result, the ununiformity of density is also significantly reduced compared to that of FIG. 26, as shown in FIG. 27C.
In such recording method, the image data is divided for the first scan and the second scan in accordance with a predetermined arrangement so that they complement each other. The image data arrangement (thinned pattern) is usually a checker pattern (or zigzag pattern) for each vertical and horizontal pixel as shown in FIG. 28. Accordingly, the printing in the unit print area (four-pixel unit in the present example) is completed by the first scan in which the checker pattern is printed and the second scan-in which the reverse checker pattern (or complementary zigzag pattern) is printed. FIGS. 28A, 28B and 28C show how the recording of a given area is completed by the checker pattern and the reverse checker pattern, when the eight-nozzle multi-head is used as it is in FIGS. 25 to 27. In the first scan, the checker pattern is recorded by using the lower four nozzles (FIG. 28A). In the second scan, the sheet is fed by four pixels (1/2 of the head length) and the reverse checker pattern is recorded (FIG. 28B). In the third scan, the sheet is further fed by four pixels (1/2 of the head length) and the checker pattern is recorded again (FIG. 28C).
In this manner, the sheet feed of the four-pixel length and the recording of the checker pattern and the reverse checker pattern are sequentially and alternately conducted so that the printing of the four-pixel unit record area is completed for each scan. As described above, since the printing in one area is completed by two different groups of nozzles, a high quality image which is free from the ununiformity of density is attained.
This printing method is disclosed in U.S. Pat. No. 4,967,203 and Japanese Laid-Open Patent Application No. 60-107975 and it is effective to solve the problems of the ununiformity of density and the connecting lines.
Although the above method can reduce the ununiformity of density due to variation in dot deposition (for example, landing deviation) and the amount of discharge, it still has a problem in that regular ununiformity of color appears when a half-tone color is printed in an entire area, due to the fact that inks of different colors are overlapped and put adjacently.
FIG. 29 shows a printing method (hereinafter referred to as an L/n sheet feed printing method, where n is the number of divisions) by the prior art head division.
In this method, the recording section (L) of the recording head is divided into two sections, and each recording head records the checked pattern or the reverse checker pattern in the first scan, and after the sheet feed by the L/2 width, it records the remaining reverse (or complementary) checker pattern or checker pattern by the different nozzles in the second scan to complete the printing. The discharge port line is not visible in the drawing but it is shown as a vertical perspective view for convenience sake.
More specifically, in the first scan, the thinned half printing of the checker pattern is conducted by the nozzles in the recording section (1) of the printing heads. Then, the sheet is fed by L/2 width. In the second scan, the thinned half printing of the reverse checker pattern is conducted by the recording heads in each of the recording sections (1) and (2). At this point, the printing by the recording section (2) is completed. The sheet is further fed by L/2 width. In the third scan, the thinner printing of the checker pattern is made for the entire area of the record area. The same steps are repeated. In FIG. 29, the indicia in the parenthes in the second and third scan indicate the previously printed ones.
A reason why the ununiformity of half-tone color takes place in the prior art method is explained below for an eight-nozzle multi-nozzle head. In this example, an image to be recorded is blanket print (or solid print) of a half tone color (yellowish green) having Cy 62.5% and Y 100% in print duty factor superimposed. The half-tone color is divided into two parts by using a checker pattern mask and a reverse checker pattern mask, and they are overprinted in two scans.
FIG. 30 shows discharge positions of a Cy recording head and a Y recording head in the first scan in the L/2 sheet feed printing method and the resulting dot formation on a recording medium. A thick hatching mark shows that Cy and Y are recorded on the same pixel. Cy dots recorded by the recording section (1) reach the sheet without any adjacent dots. In the first scan, the recording head uses four nozzles of the recording section (1) to discharge the ink in the checker pattern so that the Cy-Y overlapped dots are formed in the checker pattern on the recording medium. Then, the L/2 width sheet feed is effected and the image area on the sheet recorded in the first scan is moved toward the recording section (2).
FIG. 31 shows the discharge positions in the second scan and the resultant dot formation on the recording medium. A dark mesh pattern in the recording section (2) shows that Cy and Y are recorded on the same pixel between the adjacent pixels in which Cy and Y or Y were printed in the first scan in the recording section (1), and a light hatching pattern shows a pixel in which only Y is printed. A dark hatching pattern in the recording section (1) is same as that in the first scan, and a light hatching pattern shows a pixel in which only Y is printed. At this time, the printing head prints the reverse checker pattern in the entire area of the recording sections (1) and (2). As a result, dots are overprinted in the recording section (2) to interpolate image data to the checker pattern dots recorded in the first scan in the first image area.
Then, the L/2 sheet feed is effected so that the first image area is moved out of the recording section and the second image area having the reverse checker pattern printed in the recording section (1) is moved toward the recording section (2) and the third image area comes into the recording section (1).
The hue (coloring or color development) which is important in the color printing is now explained. When a dot is overlapped on a previously recorded dot, there is a trend that the later printed dot sinks more deeply in the depthwise direction of the sheet than the early printed dot and it spreads around the early printed dot. The same is true when a dot reaches on an adjacent dot. FIG. 33 shows a sectional view for illustrating the spread of the ink when a new dot is printed on a previously deposited dot. Pigments such as dyes in the discharged ink physically and chemically couple with the recording medium but since the coupling of the recording medium and the pigments is definite, the coupling of the early discharged ink pigments and the recording medium is preferential unless there is a big difference in the coupling force depending on the type of pigment so that the early discharged ink pigments remain on the surface of the recording medium more than the later printed ink pigments, which are hard to couple on the surface of the recording medium and sink in the depthwise direction of the sheet.
In FIG. 31, the Cy-Y mixed pixel which is heavily related to the density is shown by a dark hatching pattern as recorded in the first scan, and by a dark mesh pattern as recorded in the second scan.
FIG. 32 shows discharge positions in the third scan and the resulting dot formation on the record medium.
At this time, the checker pattern which is complementary to that in the second scan is printed in the entire area of the recording sections (1) and (2). As a result, dots deposited in the recording section (1) adjacently to the reverse checker dots recorded in the second scan to complete the printing.
Similarly, other image areas are sequentially printed by the two-pass scan by the recording head in the recording sections (1) and (2).
However, between the second image area in which the printing is completed in the third scan and the first image area in which the printing is completed in the second scan, the hues may be different in spite of the fact that the same amount of ink is ejected, and the ununiformity of color may result.
This is because a difference in the shapes of the dots having connection with the hue of the new dots which are ejected onto the image area having adjacent dots previously recorded thereon and the dots ejected onto the image area having nothing recorded thereon appears as a difference of hue between the image areas since the numbers of dots injected to the respective image areas in each scan are different. In the present example, since the numbers of Cy dots ejected to the respective image areas in the respective recording scans are different, the numbers of Cy dots which are ejected to the areas having the Cy or Y dots formed adjacently thereto to form indefinite shape dots are different between the two image areas, and hence a difference in hue appears. In the present example, the numbers of the Cy-Y mixed pixels in the respective image areas differ significantly between the first image area (hatching: 16, mesh: 4) and the second image area (hatching: 4, mesh: 16).
As explained above, in the prior art L/n sheet feed printing method, when the printing is made by a plurality of recording scans in the same image area, the improper ununiformity of color takes place in the mixed color recorded area because of the difference in the number of dots ejected in each recording scan, and this causes the degradation in the color recorded image.
Further, in the prior art L/n sheet feed printing method, substantially double number of recording scans is needed to solve the problem of the ununiformity of density due to the variation in the nozzles, and this causes the reduction of the recording speed. Thus, reciprocal scan recording may be considered to improve the recording speed but there exists a problem of difference in hue due to a difference in the sequence of ejection of the ink between the forth run and the back run, which is a problem inherent to the color ink jet recording. The reciprocal scan recording has been put into practical use in a monochromatic ink jet recording apparatus, but in the color recording, the difference in the hue between the image areas appears because the sequence of ejection the inks in the color mixed areas is different between the forth scan and the back scan. Accordingly, there is few case of the color printing which has been put into the practical use. This is due to the fact that the later ejected ink spreads around and depthwise of the early ejected dot as described above. Japanese Laid-Open Patent Application 58-194541 discloses a recording method for relieving the difference in hue in the reciprocal recording. In this method, a smaller number of dots than a total number of dots to be recorded in at least one of each row and each column of a recording dot matrix are intermittently recorded for each color in a forth pass of the reciprocal recording scan, and the remaining dots for each color are intermittently recorded in the back pass so that the dots having different sequences of ink overlap are mixed together.
This method is effective to a blanket print image in which dots of respective colors are finely recorded in a given area, but in the half-tone image data which requires the area gradation recording, the image to be recorded is inherently thinned by a predetermined gradation pattern because of the gradation representation of the image to be recorded. As a result, the gradation pattern and the thinned pattern of each reciprocal recording scan may interfere with each other to create the ununiformity of color as described above. Further, the ununiformity of density due to the variance in the nozzles is not resolved in spite of the reciprocal two-run recording scan to the given image area. Accordingly, this method cannot be simply applied and it has not been put in practical use.